This invention relates to a method of preparing screens suitable for use in screen process printing and to the preparation of photographic stencils from the screens so prepared.
Most photographic stencils used in screen process printing are prepared by either of two general methods, the transfer method and the direct method. In both methods, the photographic process of forming the stencil is essentially the same and involves placing a photograhic positive or negative, which has the image or design on it to be reproduced, against a photosensitive emulsion layer and exposing the whole unit to a source of light, whereby the emulsion is hardened in those areas struck by light. Those portions of the emulsion not struck by light remain soluble and are removed, for example, by water spray, to thereby provide the desired image or design in the emulsion. The two methods differ, however, in the technique by which the emulsion layer is provided on the screen fabric for use in printing.
In the transfer method, a photosensitive emulsion layer is carried on a support or backing sheet during the photographic exposure and washout when the image or design is formed in the emulsion. The emulsion layer, while in a moist and softened condition, is then placed in pressure contact with the screen fabric and dried, whereby the emulsion layer adheres to the fabric. Upon removal of the backing sheet, the stencil is ready for use.
In the direct method, the photosensitive emulsion is coated directly on the screen to fill the mesh of the screen and provide a layer of emulsion over the surface of the screen. After drying, the screen is processed photographically in the manner described above to provide a stencil having the desired image or design.
While the transfer method and the direct method are widely used, both methods have inherent disadvantages which adversely affect their use in commercial operations. For example, since screens prepared by the direct method are manually coated with emulsion, one at a time, it is difficult to maintain a uniform and reproducible thickness of the emulsion coating. Consequently stencils made using such emulsion coated screens have a surface which is irregular in thickness. Moreover, since the emulsion coated on the screen dries with no external support, the emulsion shrinks in all directions upon drying. Thus, in addition to shrinking into the screen the horizontal dimension of the emulsion layer shrinks in such a way that the edge surface tension effects cause a pulling of the emulsion back to the screen filaments. As a result, the edge of a stencil prepared by the direct method frequently is serrated or saw-toothed rather than a straight line. While stencils prepared by the transfer method are not subject to these disadvantages, such transfer method stencils lack the durability of stencils prepared by the direct method, since the emulsion layer is attached essentially only to the surface of the screen, with the screen fabric being embedded in the emulsion layer to a depth of only about 0.0003 - 0.0005 inch (0.3 - 0.5 mils).
In recent years another technique, a direct-film method has been suggested for use in preparing photographic stencils. According to this technique, a dry screen is placed on top of and in perfect contact with an unsensitized dry emulsion layer carried on a backing sheet. A limiting amount of photosensitive liquid emulsion is then squeegeed through the screen so that the liquid emulsion contacts the dry emulsion layer and secures it to the surface of the screen in its original thickness. After drying, the backing sheet is removed and the stencil is prepared by conventional photographic techniques. However, since the intent of this procedure is to prevent embedding of the screen in the dry emulsion layer, the resulting stencils lack the durability of stencils prepared by the direct method. Also, since the screen must be in perfect contact with the entire surface of the dry emulsion layer, this method is difficult and inconvenient to use, particularly when large stencils must be prepared.